Principles of Yacht Design

Its Loa/Bmax is thus 3.25. For an Lwl in the light condition of 9.85 m this corresponds very well with the median line of Fig 5.32. In fact, for a new design the hull is slightly narrow, since new hulls are often a bit beamier than the median according to the figure. The data in the statistical analysis of this section may be considered representative of the yacht fleets in Europe and the United States in the early 1990s, and may therefore represent an average of design trends in the 1980s and to some extent in the 1970s also.

Length of water tin e/draft (LyyL/T)

LWI/T is plotted versus Lwl in Fig 5.jj. Obviously, this ratio increases with length as well. A larger yacht has a larger ratio, ie a smaller relative draft. In fact, beam is a better scaling factor than length for the draft of a sailing yacht, and a good approximation is BMAX = 1.6 - T, which is valid more or less for all lengths. This relation corresponds very well to the median line in Fig 5.33. The upper and lower limits in this case are 15% from the median line.

Fig 5.33 Length/draft ratio

The choice of draft for a cruising yacht is a trade-off between performance and practical advantages, like the possibilities of entering more shallow water areas, ease of handling ashore etc, while for a racing yacht draft is penalized to cancel the performance advantage. The YD-40 has an Lwl of 9.85 m and a draft of 2.04 m in the light condition. This yields an Lwl/T of 4.83. According to Fig 5.33, the median for this size is 5.2, which yields a draft of 1.89. The extra 0.15 m will give the YD-40 an edge upwind, consistent with the desire to create a fast cruiser/racer.

Length of Since most modern yachts have fin keels it is possible in most cases to water line/canoe body define the canoe body draft. This seems to scale very well with length, draft (Lwl/TJ as can be seen in Fig 5.34. A typical value of Lw,/Tc is 18 for a medium displacement yacht. The ultra light dinghy type racing

machines may reach values up to 26, while heavy displacement, narrow hulls may have as small an Lwl/Tc as 12. For the ultra light hulls data are available only for large waterline lengths. The YD-40 has an Lwl/Tc close to the medium.

As explained above the length/displacement ratio is a very important

Fig 5.34 Length/hull draft ratio

Length/displacement ratio (Lwl/V'a)

quantity for the resistance of the yacht at high speeds. To enable the yacht to exceed a Froude number of about 0.45, ratios above about 5.7 are required. In Fig 5.35 the length/displacement ratio is plotted versus waterline length.

Since beam and draft do not increase linearly with length, displacement

Fig 5.35

Length/displacemenl ra tio

increases slightly slower than length cubed. In fact, with the same assumptions as above, the displacement increases as (length)7/?, which means that the length/displacement ratio increases as (length)2/». Increasing the length by a factor of two increases the ratio by 17%. The increase is not quite as fast in the statistical data, as may be seen in Fig 5.35.

As was the case in the length/beam ratio the spread is asymmetric. The lower limit in this case is some 12% below the median line, while the upper limit is put 20% above the median. There "are.- however, certain kinds of hulls outside the limits. Thus, some extreme ultra light yachts have considerably higher ratios, and since the statistics are based mainly on yachts which may participate in some kind of racing (performance handicapping system, IMS or 10R), some heavy cruising yachts may have been missed.

The length/displacement ratio is, of course, quite different between a racer and a cruiser, since the equipment required for comfortable living on board is rather heavy. In the case of the YD-40 we have tried to create a cruiser/racer with full comfort. Its length/displacement ratio is 5.16, which is close to the median for a 10 m Lwl yacht.

The overhangs of modern hulls have decreased as compared to hulls designed before the 1960s. To a certain extent this is a matter of fashion, but there is also an attempt to reduce the longitudinal gyradius as much as possible for a given (effective) waterline length. The inverse slope of the transom is another effect of this effort.

A medium value of Loa/Lwl for modern yachts is 1.23 with a spread of 0.15 up and down. There is no discernible trend with hull length. The YD-40 is very close to the median: L0A/LW, is 1.22.

Freeboard height It is a well-known fact that the relative freeboard height decreases with hull length. Obviously this is due to the requirements of the accommodation. Even on very small yachts headroom for moderately tall people is required. The trend is shown in Fig 5.36, which shows the freeboard forward versus the waterline length. No upper and lower limits arc given, since the statistical basis for this graph is smaller than for the others above (only about 50 yachts).

A typical value of freeboard forward/freeboard aft is 1.3. As compared to older yachts this is lower, so modern yachts have a more horizontal sheer line. Both the forward and aft freeboards are higher however, and the camber of the sheer line, the 'spring', is smaller. The YD-40 is representative of modern cruiser/racers and has somewhat higher freeboards than the statistical mean value, which is influenced to a certain extent by some older designs. The freeboard forward/ waterline length is 0.144, while the mean value is 0.138 for this size of hull, and the ratio of the two freeboards is 1.22.

Length overall/length of waterline

Ballast ratio

The ballast ratio, ic the ratio of keel weieht to total weieht, varies

considerably on modern yachts. A good average value is 0.45 and most yachts lie within the range 0.35-0.55 (see Fig 5.37). There docs not seem

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